The present invention relates to paper manufacturing and more particularly to an apparatus and method for sensing and controlling the tension of a paper web during the paper manufacturing process.
The tension in a web of paper as the paper is being manufactured is a critical parameter. The maintenance of proper paper sheet tension plays a major role, for example, in determining the "squareness" of the paper sheet. The squareness of paper is the ratio of the physical properties of the paper in the direction in which the paper travels through its manufacturing process to those properties in the direction substantially perpendicular to the direction of travel. Proper squareness is critical for many end uses of paper, such as xerography and other graphic applications. Paper sheet tension also has a major effect on the structure of the paper roll which is produced at the end of the paper manufacturing machine. Strict control of the structure of the paper roll is necessary to enable subsequent processing, for example on converting equipment such as rewinders and sheeters, to be accomplished.
Many grades of paper are very tension sensitive. Major product quality improvements would be possible through the measurement of actual sheet tension during the paper manufacturing process. To date, however, economical and reliable means for measuring such tension have been unavailable. Precise tension control is difficult in the absence of means for measuring the actual tension during the paper manufacturing process.
Tension control has previously been accomplished by controlling the relative speeds of the various independently driven sections of a paper machine. Unfortunately, this method of tension control can result in considerable variation in sheet tension due to changes in ambient environmental conditions, such as changes in humidity. Thus, while it is possible to control tension by varying the speed of travel at different points along a paper web, it is not always possible to maintain the tension without a means for continuously monitoring its value.
Past attempts to directly measure paper tension have involved the use of load cells which determine the load on the turning rolls in a paper manufacturing machine. This approach has not been found to be particularly useful on full size production machinery, however, because the mass of the required turning rolls acts to limit the system's sensitivity.
Accordingly, it would be advantageous to provide an apparatus and method for determining the sheet tension in a paper manufacturing machine in a dynamic manner. Accurate measurements of sheet tension could be used in conjunction with suitable data analysis and process control means to provide direct control of the sheet tension as paper is being manufactured. Such tension monitoring and control would yield considerable benefits in paper quality.
This invention relates to such an apparatus and method for tension measurement and control.